Exemplary embodiments of the present invention relate to enhanced techniques of a field effect type semiconductor device such as a metal oxide semiconductor (MOS) transistor.
The related art includes research and development of techniques to form a thin film transistor with high current drive capability that uses a crystalline semiconductor film (such as a polysilicon film) formed by low-temperature processes. An amorphous silicon film is crystallized by the application of laser beams, and therefore a polysilicon film is formed. The polysilicon film formed in this way, however, has a tendency to grow boundaries between crystal grains in various places in crystallizing (grain boundaries) that rise and become protrusion-like shapes, and therefore the unevenness of the surface becomes large. In a field effect thin film transistor wherein a gate dielectric film and a gate electrode are formed on or above this polysilicon film, an electric field concentrates at protrusions on the surface of the polysilicon film, and therefore a dielectric breakdown is likely to occur in the gate dielectric film. Related art document Japanese Unexamined Patent Publication No. 2000-40828 discloses the technique that, after forming a polysilicon film, its surface is planarized by polishing to avoid the dielectric breakdown of a gate dielectric film in a thin film transistor.
When the film thickness of a gate dielectric film is made thinner in order to promote miniaturization of a thin film transistor, the gate dielectric film is likely to have a thin film thickness at the edge of a semiconductor film. This tendency is notable when the gate dielectric film is formed using the film formation method with low step coverage such as a sputtering method or a chemical vapor deposition (CVD) method. If the gate electrode is formed to cross this edge, an electric field concentrates at the portion of crossing, thereby often leading to a dielectric breakdown. This causes a disadvantage that the reliability of a thin film transistor decreases. It is difficult in the above described related art technique to accomplish the decrease of the concentration of the electric field at the edge of the semiconductor film, and therefore further enhanced techniques have been desired.